1. Field of the Invention
The present invention relates to a transmission mechanism in a driving means for the needle of sewing machines and, more particularly, to a system for joining which is guided by a needle guide rod, to an oscillatory arm of a mechanism for causing exactly linear movements via a horizontal shaft fitted in the needle holder. The present invention provides a transmission mechanism in which the horizontal shaft is fitted in a passage in the needle holder, which horizontal shaft is provided with a vertical passage of a diameter large enough to prevent the needle guide rod passed therethrough from being slided on the inner surface thereof so that the area of slide surface of the needle holder with respect to the needle guide rod is kept uniform.
2. Description of Prior Arts
In general, the transmission mechanisms in a needle driving means of this kind are classified into a mechanism for causing nearly linear movements by which a portion where the needle holder and the oscillatory arm are joined together is moved along an arcuate line which is almost straight, and a mechanism for causing exactly linear movements by which the needle holder is moved along an exactly straight line.
A mechanism for causing nearly linear movements is designed on the basis of strict calculations of the linking ratio and errors in the linear movements of a portion where the needle holder and the oscillatory arm are directly connected to each other are proportional to the accuracy of the parts constituting the links. Therefore, the accuracy of each part requires to be absolutely perfect and the cost of manufacturing the mechanism necessarily becomes high. On the other hand, among the mechanisms for causing exactly linear movements, there is one in which the needle holder fitted on the needle guide rod is connected to the front end of a slidable shaft coaxially and slidably fitted in the oscillatory arm so that the needle guide rod is positioned on the extension line of the axis of the slidable shaft (as shown in, for example, Japanese Patent Publication No. 5422/1971).
According to the above publication, the portion where the needle guide rod, slidable shaft and needle holder are joined together and the axis of the oscillatory arm are on the same straight line and the above-mentioned joining portion is between the needle guide rod and the oscillatory arm. Consequently, when the needle holder is upwardly moved, a bending moment is exerted on the axis line of the needle guide rod and the frictional resestance due to the sliding of the needle holder on the guide rod increases. Then, there is the possibility that seizure occurs due to the heat generated during a high-speed operation of the sewing machine. The increase in frictional resistance causes wear of the parts and the durability of the machine lowers. In addition, errors occur in the positions at which the vertical movements of the needle are started and stopped and, in hemstitch sewing machines, the intersection of the needle and looper is sometimes in an improper position. Among the mechanisms for causing exactly linear movements, there is another one which includes a needle holder having longitudinal, transverse and vertical passages, a horizontal shaft having longitudinal and vertical crucial passages and fitted in the transverse passage of the needle holder, an oscillatory arm having at the front end portion thereof a vertical passage and fitted in the longitudinal passage of the horizontal passage, and a needle guide rod passed through the vertical passages in the needle holder, horizontal shaft and oscillatory arm, to vertically move the needle holder by the movement of the oscillatory arm (as shown in, for example, Japanese Patent Publication No. 5946/1974). However, the above mechanism is of a complicated construction and cannot be manufactured at a low cost. Moreover, the oscillatory arm has to be made thick so as to be sufficiently strong since it has a vertical passage through which the needle guide rod is passed. The horizontal shaft is formed outside the oscillatory arm and the needle holder is formed outside the horizontal shaft. Consequently, the mechanism as a whole is forced to be a large-sized one, which has an increased weight and an increased inertia of movement. As a result, it is not suitable for a high-speed operation of sewing machines. Among the mechanisms for causing exactly linear movements, there is still another one which includes a needle holder provided at the left and right sides thereof with a projection, and an oscillatory arm having in the front end portion thereof an elongated passage with which the projections of the needle holder are engaged in the form of a pin joint (as shown in, for example, Japanese Patent Publication No. 5946/1974). In the above mechanism, the area of the surface of the needle holder in contact with the surface of the needle guide rod is not sufficiently large. Moreover, since the needle holder slidingly contacts the needle guide rod, the contact portions thereof are greatly worn and then, the mechanism has a poor durability.